The present invention relates to a variable length frame transmission method, a variable length frame transmitter, and a variable length frame receiver.
In multimedia communication, information constituted of variable length frames such as encoded picture and sound information or multiplexed information constituted of them is transmitted.
When transmitting variable length frames from a transmitter to a receiver, frames are inevitably received by a receiver at irregular time intervals. Therefore, the receiver determines the heading or trailing portions of frames in received signals and fetches frames out of received signals. This control method for receiving variable length frames is called frame synchronism establishment.
As this type of the conventional frame synchronization method, there are a fixed length frame synchronization method (pseudo length-fixing method) and a variable length frame synchronization method.
The fixed length frame synchronization method is a method for transmitting variable length frames by mapping them to fixed length frames. This method has an advantage that, once a receiver can secure synchronism, thereafter the receiver can predict positions of the start points of successively-incoming fixed length frames and maintain a synchronous state. In the case of this method, however, the data of original variable length frames are mapped to fixed length frames, and the fixed length frames are transmitted. Therefore, it is necessary to transmit fixed length frames including a pointer showing the boundary of variable length frames in the fixed length frames. Moreover, for the receiver to completely regenerate variable length frames, it is necessary to normally receive the pointer. Therefore, it is necessary to include an error correction code or the like for protecting the pointer from a code error in a fixed length frame. Therefore, there is a problem that a frame becomes redundant and the transmission efficiency is deteriorated. Moreover, because this method maps the information for variable length frames to fixed length frames, there is a problem that the flexibility of the entire system is deteriorated.
The variable length frame synchronization method is a synchronization method assuming that a variable length frame is directly transmitted without changing the variable length frame to pseudo fixed length.
A synchronization method using a synchronization flag is typical one of variable length frame synchronization methods. In this method, a transmitter adds a synchronization flag to the head of a variable length frame and transmits the variable length frame. A receiver detects a synchronization flag out of serial data received from the transmitter and then determines the position of a variable length frame following each synchronization flag. According to this type of the variable length frame synchronization method, variable length frames are directly transmitted. Therefore, problems do not occur that a frame becomes redundant and system flexibility is deteriorated.
As an example of the variable length frame synchronization method, there is the HDLC framing adapted to ITU-T Recommendation H. 223. In the HDLC framing, a receiver establishes synchronism by adding synchronization flag frame data of 01111110 to the head of each transmitted frame and transmitting the frame.
Moreover, data transmission for mobile communication is performed under a condition in which code errors frequently occur. When adding a synchronization flag to a frame and transmitting the frame under the above condition, a burst-like code error occurs during the transmission and the synchronization flag may be broken due to the code error. In this case, because a receiver cannot detect the synchronization flag, it cannot correctly receive the frame. This trouble is referred to as miss synchronization.
Furthermore, during transmission of a frame, data that does not originally serve as a synchronization flag may be changed to a synchronization flag. In this case, a receiver erroneously recognizes the data that is not a synchronization flag as a synchronization flag and handles the data following the erroneously-recognized synchronization flag as a frame. This trouble is referred to as wrong synchronization. Moreover, this type of the erroneous synchronism is caused by the fact that a code error occurs during transmission of a frame and moreover, it occurs when the data in a frame matches with a synchronization flag even for a case in which no code error occurs.
In this case, if no burst code error occurs during transmission but only random code errors occur, it is possible to prevent the above troubles to a certain extent. That is, it is possible to reduce the miss synchronization occurrence rate by using a method that a receiver decides the data having only bit errors in an allowable range in comparison with an actual synchronization flag as a synchronization flag. In the case of actual transmission, however, not only random code errors but also burst code errors frequently occur. Therefore, it is frequent to generate a code error exceeding the above allowable range as a synchronization flag during transmission. In this case, a receiver cannot establish synchronization.
As described above, the conventional variable length frame synchronization method using a synchronization flag has an advantage that a variable length frame can be directly transmitted but it has problems that miss synchronization or wrong synchronization easily occurs in a receiver due to a code error generated during transmission and it is difficult to normally regenerate a frame when the miss synchronization or wrong synchronization occurs.
Moreover, in the case of a variable length frame transmission system, it is generally and frequently performed to transmit the information concerned with the structure of a variable length frame by including the information in the variable length frame. This type of the information is very important information to regenerate the information superimposed on a variable length frame and it is requested to enhance the protection similarly to the case of a synchronization flag. However, a variable length frame transmission art meeting the above request is not provided yet.
The present invention is made to solve the above problems and its first object is to provide a variable length frame transmission method, a transmitter, and a receiver, capable of easily establishing synchronism at the receiver even under a condition in which code error easily occurs without deteriorating the flexibility and transmission efficiency of a system.
It is the second object of the present invention to provide a variable length frame transmission method, a transmitter, and a receiver, in which the information concerned with the structure of a variable length frame is protected when transmitting the information by including it in the variable length frame.
In a variable length frame transmission method of the present invention, a transmitter divides each variable length frame to be transmitted into a plurality of code strings according to a certain division rule, adds synchronization flags to the code strings, and transmits serial data constituted of a plurality of code strings and synchronization flags added to the code strings. A receiver receives the serial data, detects the synchronization flags from the serial data, decides start points of variable length frames in the serial data based on positions of at least two synchronization flags in the serial data, and fetches the variable length frame from the serial data.
Therefore, the present invention makes it possible to accurately and easily establish synchronism without causing a frame to be redundant even under a condition in which a code error easily occurs in a synchronization flag.
In another mode of the present invention, a transmitter transmits the serial data by arranging additional information including the information concerned with the structure of a variable length frame after each synchronization flag.
Therefore, this mode has an advantage that a receiver can easily obtain additional information because it is only necessary that any one of pieces of additional information following to each synchronization flag is normally transmitted.
In another mode of the present invention, a transmitter encodes the additional information including the information concerned with the structure of a variable length frame to generate identification code strings, and arranges the identification code strings after the synchronization flags to transmit the serial data, and a receiver detects the synchronization flags and the identification code strings from the serial data, compares the identification code strings detected together with the synchronization flags and thereby decides whether each synchronization flag is included in the same variable length frame.
According to the above mode, it is possible to decide whether or not synchronization flags before identification code strings are included in the same variable length frame. Therefore, it is possible to prevent an erroneous decision from being made on the start point of a frame.